Heat exchangers are used for a wide variety of applications; for instance, in refrigeration cycle evaporators and the like. When heat exchangers are required to process two-phase fluids, special design considerations must be taken to ensure satisfactory operation in an all-gravity environment.
For instance, multiple spiral flow channels separated by suitable channels for the heat sink or heat source fluid may be used to effect a compact, lightweight, economic design. In some instances, a two-phase fluid might enter and leave from the outside of the spirals. This is effected by reversing the inner moving spiral flow channel near its center and forming an outer moving spiral channel such that there is, in effect, two spirals running counter to each other in the same plane.
The reversing spiral concept, while possessing considerable utility for applications where little temperature change of a two-phase flow is anticipated, may perform adversely in other situations. For instance, at the center of the spiral where the fluid changes radial direction, the secondary flow pattern which is fundamental to establishing annular flow, will be disturbed and control of the liquid film may be lost. Due to the confined space for spiral reversal, a pressure drop gradient will be accompanied by a temperature drop. This temperature difference between adjacent spiral passages, in addition to any associated with the presence of superheated vapor or subcooled liquid, will cause undesirable heat transfer between two-phase fluid streams in the radial direction.
In addition, if an all-gravity vapor compression refrigeration cycle evaporator of two-phase flow is desired, it not only is necessary to address the heat exchanger design, but it also is necessary to ensure uniform distribution of the fluid to each of the multiple spiral channels. Using just any approach to throttling the liquid refrigerant across an expansion valve to a low quality two-phase condition before delivering it to the heat exchanger, will subject the fluid to gravity forces which will tend to separate the liquid and vapor phases. By low quality is meant a high liquid content. The liquid will be forced in the direction of the gravity vector such that, under some conditions, a number of flow channels will be fed saturated liquid refrigerant while the majority of the flow channels will be supplied with saturated vapor.
This invention is directed to providing a compact core, multiple spiral flow evaporator or heat exchanger with a centrally located integral expansion valve where the fluid (liquid refrigerant) is introduced axially at the center and flows as a two-phase fluid in a spiral pattern to the outside. The invention ensures uniform distribution to each flow channel at all attitudes and "g" levels while alleviating the problems which may be encountered with a reverse spiral flow approach.